For some time it has been recognized that chlorinated phenols, and especially polychlorinated phenols, are susceptible to degradation by heat. Such degradation may be accelerated by contact with or exposure to scale, rust, metals, light, air, and chemical impurities.
Much has been written about the negative effects of this degradation, the focus being on the undesirability of color formation in the polychlorinated phenols. Pentachlorophenol is a well-known wood preservative agent; degraded pentachlorophenol exhibits a characteristic dark reddish or brownish color which causes discoloration in wood products to which it is applied. Thus, U.S. Pat. Nos. 3,909,365 and 4,016,047 disclose certain free radical-acting substances that may be added to polychlorinated phenols such as pentachlorophenol to act as scavengers for the color-forming impurities thereby permitting removal of the impurities from the chlorinated phenols during distillation. The compounds disclosed include certain phenols, hydroquinones, sulfur compounds, alkyl phosphites, amines, and other specific agents including zinc dust, and sodium borohydride. These additives are said to react with the color-forming impurities in the polychlorinated phenols to render them separable on distillation.
Concern has recently begun to focus on certain impurities such as the various chlorinated dibenzo-p-dioxins, hereinafter referred to by their common name as "chlorodioxins", which may be toxic to humans, possibly even in the low concentrations, e.g., about 2,000 parts per million, present in technical grade pentachlorophenol. These chlorodioxin impurities, especially octachlorodibenzo-p-dioxin and some hexachlorodibenzo-p-dioxin, are apparently formed when phenol is chlorinated to produce pentachlorophenol, and also subsequently when pentachlorophenol is heated for any reason, e.g., to be processed into a commercially salable form or in ultimately being used in the molten state. If the health hazard of the chlorodioxins is established then it will be necessary to remove them or reduce their concentration in pentachlorophenol to a safe level (i.e., below about 100 p.p.m.), and to maintain such a low level of contamination throughout subsequent processing, particularly where the pentachlorophenol is heated.
Other degradation products commonly found in pentachlorophenol that have been identified include isomers of chlorinated diphenyl ethers, chlorinated benzenes and isomers of chlorinated dibenzofurans. These compounds may be sources of additional health hazards.
This toxicity problem becomes particularly difficult to avoid when it is necessary to heat impure pentachlorophenol, such as technical grade pentachlorophenol, above its boiling point in order to purify it by distillation. Pentachlorophenol is a high-boiling solid, pure pentachlorophenol having a melting point of 190.degree. to 191.degree. C. After distillation, which is commonly conducted under reduced pressure at a temperature between about 185.degree. and about 235.degree. C., the distillation product must ordinarily be kept hot so as to keep it liquid until it can be converted into the desired final form, e.g. blocks, pellets, flakes, prills, etc. Alternatively the distillation product may be cooled and collected as a solid to be remelted later to convert it into the desired final form.
Purified pentachlorophenol can be safely heated in glass or stainless steel at 204.degree. C., its remelt temperature, for at least three hours without an increase in chlorodioxins. However, at 232.degree. C. (450.degree. F.), purified pentachlorophenol cannot be heated even one hour in stainless steel or Monel metal without build-up of chlorodioxins or other decomposition products.
Numerous compounds and compositions have been disclosed in the prior art as stabilizers for polychlorinated phenols. In particular, previously disclosed compounds and compositions include: a mixture comprising an aromatic o-hydroxycarboxylic acid, a polycarboxylic acid and an ester of a phosphorus acid, such as an alkyl or aryl phosphite (U.S. Pat. No. 3,403,186); a hydroxybenzaldehyde such as salicylaldehyde (U.S. Pat. No. 3,770,835, U.S. Pat. No. 3,852,160 and U.S. Pat. No. 3,852,161); a high boiling amine or alkanolamine (U.S. Pat. No. 3,816,268); and hydroxyl- or polyhydroxyl-containing organic compounds including sugars, polyhydric alcohols, polyglycols and ethers of polyglycols (U.S. Pat. No. 4,142,943).